Catalytically active particulate filter

ABSTRACT

The invention relates to a particulate filter which comprises a wall flow filter of length L and two different catalytically active coatings Y and Z, wherein the wall flow filter comprises channels E and A that extend in parallel between a first and a second end of the wall flow filter and are separated by porous walls which form surfaces OE and OA, respectively, and the channels E are closed at the second end and the channels A are closed at the first end. The invention is characterised in that the coating Y is located in the channels E on the surfaces OE and the coating Z is located in the channels A on the surfaces OA.

The present invention relates to a catalytically active particulate filter that is particularly suitable for the removal of particles, carbon monoxide, hydrocarbons and nitrogen oxides from the exhaust gas of internal combustion engines fueled by a stoichiometric air-fuel mixture.

Exhaust gases from combustion engines, i.e. gasoline engines, fueled by stoichiometric air/fuel mixtures are cleaned in conventional methods with the aid of three-way catalytic converters. Such catalytic converters are capable of simultaneously converting the three major gaseous pollutants of the engine, namely hydrocarbons, carbon monoxide and nitrogen oxides, into harmless components.

In addition to such gaseous pollutants, the exhaust gas from gasoline engines also contains extremely fine particles (PM), which arise from the incomplete combustion of the fuel and essentially consist of soot. In contrast to the particle emission of diesel engines, the particles in the exhaust gas of stoichiometrically operated gasoline engines are very small and have an average particle size of less than 1 μm. Typical particle sizes range from 10 to 200 nm. Furthermore, the amount of particles emitted is very low and ranges from 2 to 4 mg/km. The European exhaust emission standard EU-6c is associated with a conversion of the limit value for such particles from the particle mass limit value to a more critical particle number limit value of 6×10¹¹/km (in the Worldwide Harmonized Light Vehicles Test Cycle—WLTP). This creates a need for exhaust gas cleaning concepts for stoichiometrically operated combustion engines, which include effectively operating equipment for removing particles.

Wall flow filters made of ceramic materials, such as silicon carbide, aluminum titanate and cordierite, have proven themselves in the field of cleaning exhaust gases from lean-burn engines, i.e. in particular diesel engines. These wall flow filters are made up of a large number of parallel channels formed by porous walls. The channels are alternately closed at one of the two ends of the filter so that channels A, which are open at the first side of the filter and closed at the second side of the filter, and channels B, which are closed at the first side of the filter and open at the second side of the filter, are formed. For example, exhaust gas flowing into channels A can only leave the filter via channels B, and must flow through the porous walls between channels A and B for this purpose. When the exhaust gas passes through the wall, the particles are retained and the exhaust gas is cleaned.

The particles retained in this manner must then be burnt off or oxidized in order to prevent a clogging of the filter or an unacceptable increase in the back pressure of the exhaust system. For this purpose, the wall flow filter is, for example, provided with catalytically active coatings that reduce the ignition temperature of soot. Applying such coatings to the porous walls between the channels (so-called “on-wall coating”) or introducing them into the porous walls (so-called “in-wall coating”) is already known. EP 1 657 410 A2 also already describes a combination of both coating types; that is, part of the catalytically active material is present in the porous walls and another part is present on the porous walls.

The concept of removing particles out of the exhaust gas using wall flow filters has already been applied to the cleaning of exhaust gas out of combustion engines operated with stoichiometric air/fuel mixtures; see, for example, EP 2042226 A2. According to its teaching, a wall flow filter comprises two layers arranged one above the other, wherein one can be arranged in the porous wall and the other can be arranged on the porous wall.

DE 102011050788 A1 pursues a similar concept. There, the porous filter walls contain a catalyst material of a three-way catalytic converter, while in addition a catalyst material of a three-way catalytic converter is applied to partial regions of the filter walls.

Further documents describing filter substrates provided with catalytically active coatings are EP 3205388A1, EP 3207977 A1, EP 3207978 A1, EP 3207987 A1, EP 3207989 A1, EP 3207990 A1 and EP 3162428 A1.

There is still a need for catalytically active particulate filters that combine the functionalities of a particulate filter and a three-way catalytic converter and at the same time allow for adhering to the limits that will apply in the future.

The present invention relates to a particulate filter which comprises a wall flow filter of length L and two different coatings Y and Z, wherein the wall flow filter comprises channels E and A which extend in parallel between a first and a second end of the wall flow filter and are separated by porous walls which form the surfaces O_(E) and O_(A), respectively, and wherein the channels E are closed at the second end and the channels A are closed at the first end, wherein the coating Y is located in the channels E on the surfaces O_(E) and extends from the first end of the wall flow filter over 55 to 90% of the length L, and contains aluminum oxide in an amount of 35 to 60% by weight, based on the total weight of the coating Y, palladium, rhodium or palladium and rhodium and one or more oxygen storage components in an amount of 40 to 50% by weight based on the total weight of the coating Y, and wherein the coating Z is located in the channels A on the surfaces O_(A) and extends from the second end of the wall flow filter over 55 to 90% of the length L and contains aluminum oxide in an amount of 30 to 50% by weight based on the total weight of the coating, palladium, rhodium or palladium and rhodium and one or more oxygen storage components in a total amount of 50 to 80% by weight based on the total weight of the coating Z.

The coatings Y and Z are three-way catalytically active, especially at operating temperatures of 250 to 1100° C. They are different from each other, but both usually contain one or more precious metals fixed to one or more carrier materials and one or more oxygen storage components. The coatings Y and Z may differ in the components they contain. For example, they may differ in terms of the precious metals they contain or the oxygen storage components they contain. However, they may also contain identical components, but such components must then be present in different quantities.

Platinum, palladium and rhodium are particularly suitable as precious metals, wherein palladium, rhodium or palladium and rhodium are preferred and palladium and rhodium are particularly preferred. Based on the particulate filter according to the invention, the proportion of rhodium in the entire precious metal content is in particular greater than or equal to 10% by weight. The precious metals are usually used in quantities of 0.15 to 5 g/l based on the volume of the wall flow filter. The porous walls of the particulate filter according to the invention are preferably free of precious metals.

As carrier materials for the precious metals, all materials familiar to the person skilled in the art for this purpose can be considered. Such materials are in particular metal oxides with a BET surface area of 30 to 250 m²/g, preferably 100 to 200 m²/g (determined according to DIN 66132). Particularly suitable carrier materials for the precious metals are selected from the series consisting of aluminum oxide, doped aluminum oxide, silicon oxide, titanium dioxide and mixed oxides of one or more of these. Doped aluminum oxides are, for example, aluminum oxides doped with lanthanum oxide, zirconium oxide and/or titanium oxide. Lanthanum-stabilized aluminum oxide is advantageously used, wherein lanthanum is used in quantities of 1 to 10% by weight, preferably 3 to 6% by weight, in each case calculated as La₂O₃ and based on the weight of the stabilized aluminum oxide. Another suitable carrier material is lanthanum-stabilized aluminum oxide the surface of which is coated with lanthanum oxide, with barium oxide or with strontium oxide.

Cerium/zirconium/rare earth metal mixed oxides are particularly suitable as oxygen storage components. The term “cerium/zirconium/rare earth metal mixed oxide” within the meaning of the present invention excludes physical mixtures of cerium oxide, zirconium oxide and rare earth oxide.

Rather, “cerium/zirconium/rare earth metal mixed oxides” are characterized by a largely homogeneous, three-dimensional crystal structure that is ideally free of phases of pure cerium oxide, zirconium oxide or rare earth oxide. Depending on the manufacturing process, however, not completely homogeneous products may arise which can generally be used without any disadvantage. In all other respects, the term “rare earth metal” or “rare earth metal oxide” within the meaning of the present invention does not include cerium or cerium oxide.

Lanthanum oxide, yttrium oxide, praseodymium oxide, neodymium oxide and/or samarium oxide can, for example, be considered as rare earth metal oxides in the cerium/zirconium/rare earth metal mixed oxides. Lanthanum oxide, yttrium oxide and/or praseodymium oxide are preferred. Lanthanum oxide and/or yttrium oxide are particularly preferred, and lanthanum oxide and yttrium oxide, yttrium oxide and praseodymium oxide, and lanthanum oxide and praseodymium oxide are more particularly preferred. In embodiments of the present invention, the oxygen storage components are particularly preferably free from neodymium oxide.

In accordance with the invention, the cerium oxide to zirconium oxide mass ratio in the cerium/zirconium/rare earth metal mixed oxides can vary within wide limits. It is, for example, 0.1 to 1.5, preferably 0.2 to 1 or 0.3 to 0.5. In embodiments of the present invention, coating Y comprises an oxygen storage component having a cerium oxide content of 20 to 40% by weight based on the weight of the oxygen storage component.

In embodiments of the present invention, coating Z comprises an oxygen storage component having a cerium oxide content of 30 to 60% by weight based on the weight of the oxygen storage component. In particular, lanthanum oxide-containing oxygen storage components have a lanthanum oxide to cerium oxide mass ratio of 0.05 to 0.5.

The coatings Y and Z usually contain oxygen storage components in quantities from 15 to 120 g/l based on the volume of the wall flow filter.

The mass ratio of carrier materials and oxygen storage components in the coatings Y and Z is usually 0.3 to 1.5, for example 0.4 to 1.3.

In embodiments of the present invention, one or both of the coatings Y and Z contain an alkaline earth compound, such as strontium oxide, barium oxide or barium sulfate. The amount of barium sulfate per coating is, in particular, 2 to 20 g/l volume of the wall flow filter. Coating Z contains, in particular, strontium oxide or barium oxide.

In further embodiments of the present invention, one or both of the coatings Y and Z contain additives, such as rare earth compounds, such as lanthanum oxide, and/or binders, such as aluminum compounds. Such additives are used in quantities that may vary within wide limits and that the person skilled in the art can determine by simple means in the specific case. These help improve the rheology of the coating, if necessary.

In embodiments of the present invention, the coatings Y and Z are different from one another, but both comprise lanthanum-stabilized aluminum oxide, palladium, rhodium or palladium and rhodium and an oxygen storage component comprising zirconium oxide, cerium oxide, lanthanum oxide, and yttrium oxide and/or praseodymium oxide.

In coating Y, the yttrium oxide content is in particular 2 to 15% by weight, preferably 3 to 10% by weight, based on the weight of the oxygen storage component. The lanthanum oxide to yttrium oxide weight ratio is in particular 0.1 to 1, preferably 0.3 to 1.

In embodiments of the present invention, the yttrium oxide content in the oxygen storage component of the coating Z is the same as or greater than the yttrium oxide content in the oxygen storage component of the coating Y, based in each case on the weight of the respective oxygen storage component.

In embodiments of the present invention, the mass fraction of yttrium oxide in the coating Y is greater than in the coating Z.

Coating Z in particular may comprise an additional oxygen storage component which contains zirconium oxide, cerium oxide, lanthanum oxide, and yttrium oxide and/or praseodymium oxide. The yttrium oxide content is in particular 2 to 15% by weight, preferably 3 to 10% by weight, based on the weight of the oxygen storage component. The lanthanum oxide to yttrium oxide weight ratio is in particular 0.1 to 1, preferably 0.3 to 1.

The praseodymium oxide content is in particular 2 to 15% by weight, preferably 3 to 10% by weight, based on the weight of the oxygen storage component. The lanthanum oxide to praseodymium oxide weight ratio is in particular 0.1 to 1, preferably 0.3 to 1.

In embodiments of the present invention, the zirconium oxide content of the yttrium oxide-containing oxygen storage component is greater in coating Z than the zirconium oxide content of the praseodymium oxide-containing oxygen storage component, in each case based on the respective oxygen storage component.

In embodiments, the coatings Y each comprise lanthanum-stabilized aluminum oxide in quantities from 35 to 60% by weight, particularly preferably to 60% by weight, and an oxygen storage component in quantities from 40 to 50% by weight, particularly preferably 45 to 50% by weight, in each case based on the total weight of the coating Y. In embodiments, the coatings Z each comprise lanthanum-stabilized aluminum oxide in quantities from 30 to 50% by weight, particularly preferably 40 to 50% by weight, and an oxygen storage component in quantities from 50 to 80% by weight, particularly preferably 55 to 80% by weight, in each case based on the total weight of the coating Z.

In advantageous embodiments of the present invention, in coating Y, the weight ratio of aluminum oxide to oxygen storage component is at least 0.7 to at most 1.5, preferably 0.8-1.2.

In advantageous embodiments of the present invention, in coating Z, the weight ratio of aluminum oxide to oxygen storage component is at least 0.3 to at most 0.8, preferably 0.4-0.6.

In preferred embodiments of the present invention, the coating Y extends from the first end of the wall flow filter over 57 to 80%, in particular 57 to 65%, of the length L of the wall flow filter. The load of the wall flow filter with coating Y preferably amounts to 33 to 125 g/l based on the volume of the wall flow filter.

In preferred embodiments of the present invention, the coating Z extends from the second end of the wall flow filter over 57 to 80%, in particular 57 to 65%, of the length L of the wall flow filter. The load of the wall flow filter with coating Z preferably amounts to 33 to 125 g/l based on the volume of the wall flow filter. A preferred embodiment relates to a wall flow filter having a coating Y with a length L of 57 to 80% starting from the first end of the wall flow filter and a coating Z with a length L of 57 to 80% starting from the second end of the wall flow filter. The total washcoat load of the particulate filter according to the invention amounts in particular to 40 to 150 g/l based on the volume of the wall flow filter.

In embodiments of the present invention, the sum of the lengths of coating Y and coating Z is 110 to 160% of the length L, preferably 115 to 140% of the length L.

In embodiments of the present invention, neither coating Y nor coating Z contains a zeolite or a molecular sieve.

In one embodiment of the present invention, the latter relates to a particulate filter which comprises a wall flow filter of length L and two different coatings Y and Z, wherein the wall flow filter comprises channels E and A which extend in parallel between a first and a second end of the wall flow filter and are separated by porous walls which form the surfaces O_(E) and O_(A), respectively, and wherein the channels E are closed at the second end and the channels A are closed at the first end, characterized in that coating Y is located in the channels E on the surfaces O_(E) and extends from the first end of the wall flow filter over 57 to 80% of the length L and contains aluminum oxide in an amount of 35 to 60% by weight, based on the total weight of the coating Y, palladium, rhodium or palladium and rhodium and an oxygen storage component in an amount of 40 to 50% by weight based on the total weight of the coating Y, wherein the oxygen storage component comprises zirconium oxide, cerium oxide, lanthanum oxide and yttrium oxide, and coating Z is located in the channels A on the surfaces O_(A) and extends from the second end of the wall flow filter over 57 to 80% of the length L and contains aluminum oxide in an amount of 30 to 50% by weight based on the total weight of the coating, palladium, rhodium or palladium and rhodium and oxygen storage components in a total amount of 50 to 80% by weight based on the total weight of the coating Z, wherein an oxygen storage component contains zirconium oxide, cerium oxide, lanthanum oxide and yttrium oxide and another zirconium oxide, cerium oxide, lanthanum oxide and praseodymium oxide.

In this case, the coating Z preferably contains two different oxygen storage components, for example. The embodiments discussed above also apply with respect to the one mentioned here.

Wall flow filters that can be used in accordance with the present invention are well known and available on the market. They consist of, for example, silicon carbide, aluminum titanate or cordierite, and have, for example, a cell density of 200 to 400 cells per inch and usually a wall thickness between 6 and 12 mil, or 0.1524 and 0.305 millimeters. In the uncoated state, they have porosities of 50 to 80, in particular 55 to 75%, for example. In the uncoated state, their average pore size is, for example, 10 to 25 micrometers. Generally, the pores of the wall flow filter are so-called open pores, that is, they have a connection to the channels. Furthermore, the pores are normally interconnected with one another. This enables, on the one hand, easy coating of the inner pore surfaces and, on the other hand, easy passage of the exhaust gas through the porous walls of the wall flow filter.

The particulate filter according to the invention can be produced according to methods known to the person skilled in the art, for example by applying a coating suspension, which is usually called a washcoat, to the wall flow filter by means of one of the usual dip coating methods or pump and suction coating methods. Thermal post-treatment or calcination usually follow. The coatings Y and Z are obtained in separate and successive coating steps.

The person skilled in the art knows that the average pore size of the wall flow filter and the average particle size of the catalytically active materials must be matched to each other in order to achieve an on-wall coating or an in-wall coating. In the case of an in-wall coating, the average particle size of the catalytically active materials must be small enough to penetrate the pores of the wall flow filter. In contrast, in the case of an on-wall coating, the average particle size of the catalytically active materials must be large enough not to penetrate the pores of the wall flow filter.

In embodiments of the present invention, the coating suspensions for the production of the coatings Y and Z are ground to a particle size distribution of d₅₀=4 to 8 μm and d₉₉=22 to 16 μm.

The particulate filter according to the invention is perfectly suitable for removing particles, carbon monoxide, hydrocarbons and nitrogen oxides out of the exhaust gas of combustion engines operated with stoichiometric air/fuel mixture.

The present invention thus also relates to a method for removing particles, carbon monoxide, hydrocarbons and nitrogen oxides out of the exhaust gas of combustion engines operated with stoichiometric air/fuel mixture, characterized in that the exhaust gas is passed over a particulate filter according to the invention.

The exhaust gas can be passed over a particulate filter according to the invention in such a way that it enters the particulate filter through channels E and leaves it again through channels A. However, it is also possible for the exhaust gas to enter the particulate filter through channels A and to leave it again through channels E.

Surprisingly, it has been found that it is advantageous to distribute the catalytic coating over the largest possible surface area of the porous filter wall. According to the experiments, the critical factor for a low exhaust back pressure is not, as originally assumed, the degree of coverage of the filter walls but rather the layer thickness of the applied catalytic coating. By distributing the coating over a large area over at least 55% of the filter length per zone, the exhaust back pressure can be reduced and a high catalytic activity can be achieved at the same time. This was not to be expected on the basis of the known prior art.

FIG. 1 shows a particulate filter according to the invention which comprises a wall flow filter of length L (1) having channels E (2) and channels A (3) that extend in parallel between a first end (4) and a second end (5) of the wall flow filter and are separated by porous walls (6), which form surfaces O_(E) (7) or O_(A) (8), respectively, and wherein the channels E (2) are closed at the second end (5) and the channels A (3) are closed at the first end (4). Coating Y (9) is located in the channels E (2) on the surfaces O_(E) (7) and coating Z (10) is located in the channels A (3) on the surfaces O_(A) (8).

The invention is explained in more detail in the following examples.

COMPARATIVE EXAMPLE 1

Aluminum oxide stabilized with lanthanum oxide was suspended in water with a first oxygen storage component, which comprised 40% by weight cerium oxide, zirconium oxide, lanthanum oxide and praseodymium oxide, and a second oxygen storage component, which comprised 24% by weight cerium oxide, zirconium oxide, lanthanum oxide and yttrium oxide. Both oxygen storage components were used in equal parts. The weight ratio of aluminum oxide and oxygen storage component was 30:70. The suspension thus obtained was then mixed with a palladium nitrate solution and a rhodium nitrate solution under constant stirring. The resulting coating suspension was used directly for coating a commercially available wall flow filter substrate, the coating being introduced into the porous filter wall over 100% of the substrate length. The total load of this filter amounted to 75 g/l; the total precious metal load amounted to 1.27 g/l with a ratio of palladium to rhodium of 5:1. The coated filter thus obtained was dried and then calcined. It is hereinafter referred to as VGPF1.

EXAMPLE 1 Coating the Input Channels

a) Aluminum oxide stabilized with lanthanum oxide was suspended in water with an oxygen storage component, which comprised 24% by weight cerium oxide, zirconium oxide, lanthanum oxide and yttrium oxide. The weight ratio of aluminum oxide and oxygen storage component was 56:44. The suspension thus obtained was then mixed with a palladium nitrate solution and a rhodium nitrate solution under constant stirring. The resulting coating suspension was used directly to coat a commercially available wall flow filter substrate. The coating suspension was coated onto the filter walls of the substrate in the input channels over a length of 60% of the filter length. The load of the inlet channel amounted to 62.5 g/l; the precious metal load amounted to 1.06 g/l with a ratio of palladium to rhodium of 5:1. The coated filter thus obtained was dried and then calcined.

Coating the Output Channels

b) Aluminum oxide stabilized with lanthanum oxide was suspended in water with a first oxygen storage component, which comprised 40% by weight cerium oxide, zirconium oxide, lanthanum oxide and praseodymium oxide, and a second oxygen storage component, which comprised 24% by weight cerium oxide, zirconium oxide, lanthanum oxide and yttrium oxide. Both oxygen storage components were used in equal parts. The weight ratio of aluminum oxide and oxygen storage component was 30:70. The suspension thus obtained was then mixed with a palladium nitrate solution and a rhodium nitrate solution under constant stirring. The resulting coating suspension was used directly to coat the wall flow filter substrate obtained under a), wherein the filter walls of the substrate were coated in the output channels over a length of 60% of the filter length. The load of the outlet channel amounted to 62.5 g/l; the precious metal load amounted to 1.06 g/l with a ratio of palladium to rhodium of 5:1. The coated filter thus obtained was dried and then calcined. The total load of this filter thus amounted to 75 g/l; the total precious metal load amounted to 1.27 g/l with a palladium to rhodium ratio of 5:1. It is hereinafter referred to as GPF1.

Catalytic Characterization

The particulate filters VGPF1 and GPF1 were aged together in an engine test bench aging process. This aging process consists of an overrun cut-off aging process with an exhaust gas temperature of 950° C. before the catalyst inlet (maximum bed temperature of 1030° C.). The aging time was 19 hours (see Motortechnische Zeitschrift, 1994, 55, 214-218).

The catalytically active particulate filters were then tested in the aged state at an engine test bench in the so-called “light-off test” and in the “lambda sweep test.” In the light-off test, the light-off behavior is determined in the case of a stoichiometric exhaust gas composition with a constant average air ratio λ (λ=0.999 with ±3.4% amplitude).

Table 1 below contains the temperatures Tso at which 50% of the considered components are respectively converted.

TABLE 1 T₅₀ HC T₅₀ CO T₅₀ NOx stoichiometric stoichiometric stoichiometric VGPF1 375 382 388 GPF1 363 367 368

The dynamic conversion behavior of the particulate filters was determined in a lambda sweep test in a range from λ=0.99-1.01 at a constant temperature of 510° C. The amplitude of λ in this case was ±3.4%. Table 2 shows the conversion at the intersection of the CO and NOx conversion curves, along with the associated HC conversion of the aged particulate filters.

TABLE 2 CO/NOx conversion at the HC conversion at λ of the point of intersection CO/NOx point of intersection VGPF1 84% 96% GPF1 96% 97%

The particulate filter GPF1 according to the invention shows a marked improvement in light-off behavior and dynamic CO/NOx conversion in the aged state compared to VGPF1.

COMPARATIVE EXAMPLE 2

a) Application of the In-Wall Coating:

Aluminum oxide stabilized with lanthanum oxide was suspended in water with a first oxygen storage component, which comprised 40% by weight cerium oxide, zirconium oxide, lanthanum oxide and praseodymium oxide, and a second oxygen storage component, which comprised 24% by weight cerium oxide, zirconium oxide, lanthanum oxide and yttrium oxide. Both oxygen storage components were used in equal parts. The weight ratio of aluminum oxide and oxygen storage component was 30:70. The suspension thus obtained was then mixed with a palladium nitrate solution and a rhodium nitrate solution under constant stirring. The resulting coating suspension was used directly for coating a commercially available wall flow filter substrate, the coating being introduced into the porous filter wall over 100% of the substrate length. The total load of this filter amounted to 100 g/l; the precious metal load amounted to 2.60 g/l with a palladium to rhodium ratio of 60:13.75. The coated filter thus obtained was dried and then calcined.

b) Coating the Input Channels

Aluminum oxide stabilized with lanthanum oxide was suspended in water with an oxygen storage component, which comprised 40% by weight cerium oxide, zirconium oxide, lanthanum oxide and praseodymium oxide. The weight ratio of aluminum oxide and oxygen storage component was 50/50. The suspension thus obtained was then mixed with a palladium nitrate solution and a rhodium nitrate solution under constant stirring. The resulting coating suspension was used directly to coat the wall flow filter substrate obtained under a), wherein the filter walls of the substrate were coated in the input channels over a length of 25% of the filter length. The load of the input channel amounted to 58 g/l; the precious metal load amounted to 2.30 g/l with a ratio of palladium to rhodium of 10:3. The coated filter thus obtained was dried and then calcined.

c) Coating the Output Channels

Aluminum oxide stabilized with lanthanum oxide was suspended in water with an oxygen storage component, which comprised 24% by weight cerium oxide, zirconium oxide, lanthanum oxide and yttrium oxide. The weight ratio of aluminum oxide and oxygen storage component was 56/44. The suspension thus obtained was then mixed with a palladium nitrate solution and a rhodium nitrate solution under constant stirring. The resulting coating suspension was used directly to coat the wall flow filter substrate obtained under b), wherein the filter walls of the substrate were coated in the output channels over a length of 25% of the filter length. The load of the outlet channel amounted to 59 g/l; the precious metal load amounted to 1.06 g/l with a palladium to rhodium ratio of 1:2. The coated filter thus obtained was dried and then calcined. The total load of this filter thus amounted to 130 g/l; the total precious metal load amounted to 3.44 g/l with a palladium to rhodium ratio of 10:3. It is hereinafter referred to as VGPF2.

COMPARATIVE EXAMPLE 3

a) Application of the In-Wall Coating:

Aluminum oxide stabilized with lanthanum oxide was suspended in water with a first oxygen storage component, which comprised 40% by weight cerium oxide, zirconium oxide, lanthanum oxide and praseodymium oxide, and a second oxygen storage component, which comprised 24% by weight cerium oxide, zirconium oxide, lanthanum oxide and yttrium oxide. Both oxygen storage components were used in equal parts. The weight ratio of aluminum oxide and oxygen storage component was 30:70. The suspension thus obtained was then mixed with a palladium nitrate solution and a rhodium nitrate solution under constant stirring. The resulting coating suspension was used directly for coating a commercially available wall flow filter substrate, the coating being introduced into the porous filter wall over 100% of the substrate length. The total load of this filter amounted to 100 g/l; the precious metal load amounted to 2.07 g/l with a palladium to rhodium ratio of 45:13.5. The coated filter thus obtained was dried and then calcined.

b) Coating the Input Channels

Aluminum oxide stabilized with lanthanum oxide was suspended in water with an oxygen storage component, which comprised 40% by weight cerium oxide, zirconium oxide, lanthanum oxide and praseodymium oxide. The weight ratio of aluminum oxide and oxygen storage component was 50/50. The suspension thus obtained was then mixed with a palladium nitrate solution and a rhodium nitrate solution under constant stirring. The resulting coating suspension was used directly to coat the wall flow filter substrate obtained under a), wherein the filter walls of the substrate were coated in the input channels over a length of 60% of the filter length. The load of the input channel amounted to 90 g/l; the precious metal load amounted to 2.30 g/l with a palladium to rhodium ratio of 10:3. The coated filter thus obtained was dried and then calcined. The total load of this filter thus amounted to 154 g/l; the total precious metal load amounted to 3.44 g/l with a palladium to rhodium ratio of 10:3. It is hereinafter referred to as VGPF3.

EXAMPLE 2 Coating the Input Channels

a) Aluminum oxide stabilized with lanthanum oxide was suspended in water with an oxygen storage component, which comprised 24% by weight cerium oxide, zirconium oxide, lanthanum oxide and yttrium oxide. The weight ratio of aluminum oxide and oxygen storage component was 56/44. The suspension thus obtained was then mixed with a palladium nitrate solution and a rhodium nitrate solution under constant stirring. The resulting coating suspension was used directly to coat a commercially available wall flow filter substrate. The coating suspension was coated onto the filter walls of the substrate in the input channels over a length of 60% of the filter length. The load of the inlet channel amounted to 83.33 g/l; the precious metal load amounted to 2.87 g/l with a palladium to rhodium ratio of 10:3. The coated filter thus obtained was dried and then calcined.

Coating the Output Channels

b) Aluminum oxide stabilized with lanthanum oxide was suspended in water with a first oxygen storage component, which comprised 40% by weight cerium oxide, zirconium oxide, lanthanum oxide and praseodymium oxide, and a second oxygen storage component, which comprised 24% by weight cerium oxide, zirconium oxide, lanthanum oxide and yttrium oxide. Both oxygen storage components were used in equal parts. The weight ratio of aluminum oxide and oxygen storage component was 30:70. The suspension thus obtained was then mixed with a palladium nitrate solution and a rhodium nitrate solution under constant stirring. The resulting coating suspension was used directly to coat the wall flow filter substrate obtained under a), wherein the filter walls of the substrate were coated in the output channels over a length of 60% of the filter length. The load of the outlet channel amounted to 83.33 g/l; the precious metal load amounted to 2.87 g/l with a palladium to rhodium ratio of 10:3. The coated filter thus obtained was dried and then calcined. The total load of this filter thus amounted to 100 g/l: the total precious metal load amounted to 3.44 g/l with a palladium to rhodium ratio of 10:3. It is hereinafter referred to as GPF2.

Catalytic Characterization

The particulate filters VGPF2, VGPF3 and GPF2 were aged together in an engine test bench aging process. This aging process consists of an overrun cut-off aging process with an exhaust gas temperature of 950° C. before the catalyst inlet (maximum bed temperature of 1030° C.). The aging time was 76 hours (see Motortechnische Zeitschrift, 1994, 55, 214-218).

The catalytically active particulate filters were then tested in the aged state at an engine test bench in the so-called “light-off test” and in the “lambda sweep test.” In the light-off test, the light-off behavior is determined in the case of a stoichiometric exhaust gas composition with a constant average air ratio λ (λ=0.999 with ±3.4% amplitude).

Table 3 below contains the temperatures Tso at which 50% of the considered components are respectively converted.

TABLE 3 T₅₀ HC T₅₀ CO T₅₀ NOx stoichiometric stoichiometric stoichiometric VGPF2 368 374 371 VGPF3 387 395 396 GPF2 332 335 332

The dynamic conversion behavior of the particulate filters was determined in a lambda sweep test in a range from λ=0.99-1.01 at a constant temperature of 510° C. The amplitude of A in this case was ±3.4%. Table 4 shows the conversion at the intersection of the CO and NOx conversion curves, along with the associated HC conversion of the aged particulate filters.

TABLE 4 CO/NOx conversion at the HC conversion at λ of the point of intersection CO/NOx point of intersection VGPF2 92 97 VGPF3 93 97 GPF2 98 98

The particulate filter GPF2 according to the invention shows a marked improvement in light-off behavior and dynamic CO/NOx conversion in the aged state compared to VGPF2 and VGPF3.

COMPARATIVE EXAMPLE 4 Coating the Input Channels

a) Aluminum oxide stabilized with lanthanum oxide was suspended in water with an oxygen storage component, which comprised 24% by weight cerium oxide, zirconium oxide, lanthanum oxide and yttrium oxide. The weight ratio of aluminum oxide and oxygen storage component was 56/44. The suspension thus obtained was then mixed with a palladium nitrate solution and a rhodium nitrate solution under constant stirring. The resulting coating suspension was used directly to coat a commercially available wall flow filter substrate. The coating suspension was coated onto the filter walls of the substrate in the input channels over a length of 50% of the filter length. The load of the inlet channel amounted to 100 g/l; the precious metal load amounted to 1.42 g/l with a palladium to rhodium ratio of 5:1. The coated filter thus obtained was dried and then calcined.

Coating the Output Channels

b) Aluminum oxide stabilized with lanthanum oxide was suspended in water with a first oxygen storage component, which comprised 40% by weight cerium oxide, zirconium oxide, lanthanum oxide and praseodymium oxide, and a second oxygen storage component, which comprised 24% by weight cerium oxide, zirconium oxide, lanthanum oxide and yttrium oxide. Both oxygen storage components were used in equal parts. The weight ratio of aluminum oxide and oxygen storage component was 30:70. The suspension thus obtained was then mixed with a palladium nitrate solution and a rhodium nitrate solution under constant stirring. The resulting coating suspension was used directly to coat the wall flow filter substrate obtained under a), wherein the filter walls of the substrate were coated in the output channels over a length of 50% of the filter length. The load of the outlet channel amounted to 100 g/l; the precious metal load amounted to 1.42 g/l with a palladium to rhodium ratio of 5:1. The coated filter thus obtained was dried and then calcined. The total load of this filter thus amounted to 100 g/l; the total precious metal load amounted to 1.42 g/l with a ratio of palladium to rhodium of 5:1. It is hereinafter referred to as VGPF4.

EXAMPLE 3 Coating the Input Channels

a) Aluminum oxide stabilized with lanthanum oxide was suspended in water with an oxygen storage component, which comprised 24% by weight cerium oxide, zirconium oxide, lanthanum oxide and yttrium oxide. The weight ratio of aluminum oxide and oxygen storage component was 56/44. The suspension thus obtained was then mixed with a palladium nitrate solution and a rhodium nitrate solution under constant stirring. The resulting coating suspension was used directly to coat a commercially available wall flow filter substrate. The coating suspension was coated onto the filter walls of the substrate in the input channels over a length of 55% of the filter length. The load of the inlet channel amounted to 91 g/l; the precious metal load amounted to 1.16 g/l with a palladium to rhodium ratio of 5:1. The coated filter thus obtained was dried and then calcined.

Coating the Output Channels

b) Aluminum oxide stabilized with lanthanum oxide was suspended in water with a first oxygen storage component, which comprised 40% by weight cerium oxide, zirconium oxide, lanthanum oxide and praseodymium oxide, and a second oxygen storage component, which comprised 24% by weight cerium oxide, zirconium oxide, lanthanum oxide and yttrium oxide. Both oxygen storage components were used in equal parts. The weight ratio of aluminum oxide and oxygen storage component was 30:70. The suspension thus obtained was then mixed with a palladium nitrate solution and a rhodium nitrate solution under constant stirring. The resulting coating suspension was used directly to coat the wall flow filter substrate obtained under a), wherein the filter walls of the substrate were coated in the output channels over a length of 55% of the filter length. The load of the outlet channel amounted to 91 g/l; the precious metal load amounted to 1.16 g/l with a palladium to rhodium ratio of 5:1. The coated filter thus obtained was dried and then calcined. The total load of this filter thus amounted to 100 g/l; the total precious metal load amounted to 1.42 g/l with a ratio of palladium to rhodium of 5:1. It is hereinafter referred to as GPF3.

EXAMPLE 4 Coating the Input Channels

a) Aluminum oxide stabilized with lanthanum oxide was suspended in water with an oxygen storage component, which comprised 24% by weight cerium oxide, zirconium oxide, lanthanum oxide and yttrium oxide. The weight ratio of aluminum oxide and oxygen storage component was 56/44. The suspension thus obtained was then mixed with a palladium nitrate solution and a rhodium nitrate solution under constant stirring. The resulting coating suspension was used directly to coat a commercially available wall flow filter substrate. The coating suspension was coated onto the filter walls of the substrate in the input channels over a length of 60% of the filter length. The load of the inlet channel amounted to 83.33 g/l; the precious metal load amounted to 1.06 g/l with a palladium to rhodium ratio of 5:1. The coated filter thus obtained was dried and then calcined.

Coating the Output Channels

b) Aluminum oxide stabilized with lanthanum oxide was suspended in water with a first oxygen storage component, which comprised 40% by weight cerium oxide, zirconium oxide, lanthanum oxide and praseodymium oxide, and a second oxygen storage component, which comprised 24% by weight cerium oxide, zirconium oxide, lanthanum oxide and yttrium oxide. Both oxygen storage components were used in equal parts. The weight ratio of aluminum oxide and oxygen storage component was 30:70. The suspension thus obtained was then mixed with a palladium nitrate solution and a rhodium nitrate solution under constant stirring. The resulting coating suspension was used directly to coat the wall flow filter substrate obtained under a), wherein the filter walls of the substrate were coated in the output channels over a length of 60% of the filter length. The load of the outlet channel amounted to 83.33 g/l; the precious metal load amounted to 1.06 g/l with a palladium to rhodium ratio of 5:1. The coated filter thus obtained was dried and then calcined. The total load of this filter thus amounted to 100 g/l; the total precious metal load amounted to 1.42 g/l with a ratio of palladium to rhodium of 5:1. It is hereinafter referred to as GPF4.

EXAMPLE 5 Coating the Input Channels

a) Aluminum oxide stabilized with lanthanum oxide was suspended in water with an oxygen storage component, which comprised 24% by weight cerium oxide, zirconium oxide, lanthanum oxide and yttrium oxide. The weight ratio of aluminum oxide and oxygen storage component was 56/44. The suspension thus obtained was then mixed with a palladium nitrate solution and a rhodium nitrate solution under constant stirring. The resulting coating suspension was used directly to coat a commercially available wall flow filter substrate. The coating suspension was coated onto the filter walls of the substrate in the input channels over a length of 80% of the filter length. The load of the inlet channel amounted to 62.5 g/l; the precious metal load amounted to 0.79 g/l with a palladium to rhodium ratio of 5:1. The coated filter thus obtained was dried and then calcined.

Coating the Output Channels

b) Aluminum oxide stabilized with lanthanum oxide was suspended in water with a first oxygen storage component, which comprised 40% by weight cerium oxide, zirconium oxide, lanthanum oxide and praseodymium oxide, and a second oxygen storage component, which comprised 24% by weight cerium oxide, zirconium oxide, lanthanum oxide and yttrium oxide. Both oxygen storage components were used in equal parts. The weight ratio of aluminum oxide and oxygen storage component was 30:70. The suspension thus obtained was then mixed with a palladium nitrate solution and a rhodium nitrate solution under constant stirring. The resulting coating suspension was used directly to coat the wall flow filter substrate obtained under a), wherein the filter walls of the substrate were coated in the output channels over a length of 80% of the filter length. The load of the outlet channel amounted to 62.5 g/l; the precious metal load amounted to 0.79 g/l with a palladium to rhodium ratio of 5:1. The coated filter thus obtained was dried and then calcined. The total load of this filter thus amounted to 100 g/l; the total precious metal load amounted to 1.42 g/l with a ratio of palladium to rhodium of 5:1. It is hereinafter referred to as GPF5.

Catalytic Characterization

The particulate filters VGPF4, GPF3, GPF4 and GPF5 were compared on a cold blow test bench with respect to the exhaust back pressure.

Table 5 below shows pressure loss data which were determined at an air temperature of 21° C. and a volume flow rate of 600 m³/h. The values were normalized to VGPF3 for better clarity.

TABLE 5 VGPF4 GPF3 GPF4 GPF5 Δp normalized to VGPF3 100 92 78 70

The filters GPF3, GPF4 and GPF5 according to the invention all surprisingly have a lower pressure loss than the comparative example VGPF4, even though they cover a larger surface of the filter walls. This is quite surprising since it could actually be assumed that longer coatings cause a higher exhaust back pressure, since here more exhaust gas must flow through the catalytic coatings, since as a result less exhaust gas can flow through the filter wall that is not provided with a coating.

It was furthermore systematically investigated what the main effects responsible for the lowest possible exhaust back pressure are. Here, various filters with different zone lengths (factor A) and washcoat layer thicknesses (factor B) were prepared and compared with one another. All filters had the same total washcoat load and the same precious metal content.

TABLE 6 Factor Name Unit Min Max A Zone length % 30 60 B Washcoat thickness g/l 50 80

The statistical evaluation shows that it is particularly advantageous to distribute the washcoat on as large a surface as possible on the filter walls with a resultant low layer thickness, instead of covering only a small surface with a high layer thickness, since a high layer thickness is to be regarded as the main cause of a high exhaust back pressure (FIG. 2). In addition, the particulate filters were aged together in an engine test bench aging process. This aging process consists of an overrun cut-off aging process with an exhaust gas temperature of 950° C. before the catalyst inlet (maximum bed temperature of 1030° C.). The aging time was 19 hours (see Motortechnische Zeitschrift, 1994, 55, 214-218).

The catalytically active particulate filters were then tested in the aged state at an engine test bench in the so-called “lambda sweep test.” Surprisingly, the statistical evaluation of the test results also shows a significant advantage in the lambda sweep test if the catalytic coating is applied with a low layer thickness to as large a surface as possible (FIG. 3).

In addition, it was investigated to what extent an embodiment consisting of one short and one long zone differs from an embodiment consisting of two long zones. For this purpose, a filter according to the invention with zone lengths of 60% of the filter length in each case was compared with a comparison filter with zone lengths of 90% in the inlet channel and 30% in the outlet channel. In the light-off test, in which the light-off behavior in the case of a stoichiometric exhaust gas composition with a constant average air ratio λ is determined (λ=0.999 with ±3.4% amplitude), it is found that the filter according to the invention with zones lengths of 60% in each case can convert the corresponding exhaust gas components at lower temperatures than the filter not according to the invention with zone lengths of 90% and 30%. Table 7 below contains the temperatures Tso at which 50% of the considered components are respectively converted.

TABLE 7 Zone T₅₀ HC T₅₀ CO T₅₀ NOx length stoichiometric stoichiometric stoichiometric 90:30 288 287 288 60:60 284 284 284 

1. Particulate filter which comprises a wall flow filter of length L and two different coatings Y and Z, wherein the wall flow filter comprises channels E and A that extend in parallel between a first and a second end of the wall flow filter and are separated by porous walls which form surfaces O_(E) and O_(A), respectively, and wherein the channels E are closed at the second end and the channels A are closed at the first end, characterized in that coating Y is located in channels E on the surfaces O_(E) and extends from the first end of the wall flow filter over 55 to 90% of the length L and contains aluminum oxide in an amount of 35 to 60% by weight based on the total weight of the coating Y, palladium, rhodium or palladium and rhodium and one or more oxygen storage components in an amount of 40 to 50% by weight based on the total weight of the coating Y, and coating Z is located in the channels A on the surfaces O_(A) and extends from the second end of the wall flow filter over 55 to 90% of the length L and contains aluminum oxide in an amount of 30 to 50% by weight based on the total weight of the coating, palladium or palladium and rhodium and one or more oxygen storage components in a total amount of 50 to 80% by weight based on the total weight of the coating Z.
 2. Particulate filter according to claim 1, characterized in that the coating Y extends from the first end of the wall flow filter to 57 over 80% of the length L of the wall flow filter.
 3. Particulate filter according to claim 1, characterized in that the coating Z extends from the second end of the wall flow filter over 57 to 80% of the length L of the wall flow filter.
 4. Particulate filter according to claim 1, characterized in that each of the coatings Y and Z contains one or more of the precious metals fixed to one or more carrier materials, and one or more oxygen storage components.
 5. Particulate filter according to claim 1, characterized in that the carrier materials for the precious metals are selected from the series consisting of aluminum oxide, doped aluminum oxide, silicon oxide, titanium dioxide and mixed oxides of one or more of these.
 6. Particulate filter according to claim 4, characterized in that the carrier materials for the precious metals are metal oxides with a BET surface area of 30 to 250 m²/g (determined according to DIN 66132).
 7. Particulate filter according to claim 4, characterized in that the coatings Y and Z contain a cerium/zirconium/rare earth metal mixed oxide as oxygen storage component.
 8. Particulate filter according to claim 7, characterized in that the cerium/zirconium/rare earth metal mixed oxides contain lanthanum oxide, yttrium oxide, praseodymium oxide, neodymium oxide and/or samarium oxide as rare earth metal oxide.
 9. Particulate filter according to claim 7, characterized in that the cerium/zirconium/rare earth metal mixed oxides contain lanthanum oxide and yttrium oxide, yttrium oxide and praseodymium oxide or lanthanum oxide and praseodymium oxide as rare earth metal oxide.
 10. Particulate filter according to claim 4, characterized in that the coatings Y and Z both comprise lanthanum-stabilized aluminum oxide, palladium, rhodium or palladium and rhodium and an oxygen storage component comprising zirconium oxide, cerium oxide, lanthanum oxide, and yttrium oxide and/or praseodymium oxide.
 11. Particulate filter according to claim 10, characterized in that coating Z comprises an additional oxygen storage component which contains zirconium oxide, cerium oxide, lanthanum oxide, and praseodymium oxide and/or yttrium oxide.
 12. Method for removing particles, carbon monoxide, hydrocarbons and nitrogen oxides from the exhaust gas of combustion engines fueled by a stoichiometric air/fuel mixture, characterized in that the exhaust gas is passed over a particulate filter according to claim
 1. 